'A universe beneath our feet': Life in Beijing's underground

© Sim Chi Yin/VII

Twenty-one-year-old Liu Jing sits in her apartment two stories underground in Beijing, China. Her laundry hangs above her head because there's nowhere else to put her clothes.

In Beijing, even the tiniest apartment can cost a fortune - after all, with more than 21 million residents, space is limited and demand is high.

But it is possible to find more affordable housing. You'll just have to join an estimated 1 million of the city's residents and look underground.

Below the city's bustling streets, bomb shelters and storage basements are turned into illegal - but affordable - apartments.

Claustrophobic Living Quarters

Annette Kim, a professor at the University of Southern California who researches urbanization, spent last year in China's capital city studying the underground housing market.

"Part of why there's so much underground space is because it's the official building code to continue to build bomb shelters and basements," Kim says. "That's a lot of new, underground space that's increasing in supply all the time. They're everywhere."

She says apartments go one to three stories below ground. Residents have communal bathrooms and shared kitchens. The tiny, windowless rooms have just enough space to fit a bed.

"It's tight," Kim says. "But I also lived in Beijing for a year, and the city, in general, is tight."

With an average rent of $70 per month, she says, this is an affordable option for city-dwellers.

But living underground is illegal, Kim says, since housing laws changed in 2010.

And, in addition, there's a stigma to living in basements and bomb shelters, as Kim found when she interviewed residents above ground about their neighbors directly below.

"They weren't sure who was down there," Kim says. "There is actually very little contact between above ground and below ground, and so there's this fear of security."

In reality, she says, the underground residents are mostly young migrants who moved from the countryside looking for work in Beijing.

"They're all the service people in the city," she says. "They're your waitresses, store clerks, interior designers, tech workers, who just can't afford a place in the city."

Kim says there's a range of units, from the dark and dingy to the neatly decorated.

But it's rare to get a glimpse below. Property owners can be strict about whom they let in.

© Sim Chi Yin/VII

Zhuang Qiuli and her boyfriend Feng Tao sit on the bed in their basement apartment two floors below a posh condominium. Since this photo was taken, the couple has moved above ground.

The 'Rat Tribe'

Beijing-based photographer Chi Yin Sim found a way. She has documented life under the city in a collection called China's "Rat Tribe."

"I started to try and find ways to get down there because I was fascinated by the fact that there was a universe beneath our feet," Sim says.

The first basement-dweller she met was a young woman, a pedicurist at a salon, who lived with her boyfriend.

"I was just like, 'Can I come and visit?' And she was like, 'Sure, come and visit us,' " Sim says.

The couple lived two floors below a posh Beijing apartment complex.

Sim's photos show just how tiny these units really are. The couple sits on their bed, surrounded by clothes, boxes and a giant teddy bear. There's hardly any room to move around.

"The air is not so good, ventilation is not so good," Sim says. "And the main complaint that people have is not that they can't see the sun: It's that it's very humid in the summer. So everything that they put out in their rooms gets a bit moldy, because it's just very damp and dank underground."

Sim says residents adapt to the close quarters.

"At dinnertime you can hear people cooking, you can hear people chit-chatting in the next room, you can hear people watching television," she says. "It's really not so bad. I mean, you're spending almost all your day at work anyway. And you're coming back, and all you need is a clean and safe place to sleep in."

© Sim Chi Yin/VII

Xie Jinghui sometimes does some weightlifting in his basement room. Photographer Sim Chi Yin says people adapt to these close quarters.

Of course, it's not ideal. Sim met a number of people who were too embarrassed to have their photo taken.

Annette Kim from USC says it's especially hard for the older residents, some of whom have been down there for years.

"They're hoping that their next generation, their children, will be able to live above ground," Kim says. "It's this sense of longing and deferring a dream. And so it makes me wonder how long this dream can be deferred."

But despite the laws against living underground, and the discomfort and shame associated with it, Kim says it's still a very active market. For hundreds of thousands of people, it's the only viable option for living in, or under, Beijing.